The present invention relates to communications devices and protocols such as those used in wireless alarm systems having multiple sensors in communication with one or more receiver control units; and in particular to such alarm systems where the transmitting element of the system is designed to transmit messages at different power levels.
Most radio frequency (RF) wireless security systems available today, such as those manufactured by Alarm Manufacturing Device Co. (ADEMCO) 165 Eileen Way, Syosset, N.Y. 11791, employ a multiplicity of transmitters in communication with a central receiver control unit. The information transmitted typically describes the state of various transducers or sensors associated with each transmitter, such as smoke, motion, breaking glass, shock and vibration detectors; door, window and floor mat switches; etc. These transmitters are designed to be inexpensive to manufacture and generally are capable of transmission only rather than reception only or transmission and reception, which would add significant cost to the design. In order to meet certain regulatory agency requirements, the transmitters must periodically transmit supervisory messages to the central receiver control unit in order to identify potential problems with the communication link from any transmitter in the alarm system as soon as possible. The supervisory message (as well as a normal alarm message) comprises a unique embedded identification code, which serves to identify to the central receiver control unit the source of the particular supervisory message (or alarm message). Typically, when a supervisory message is properly received and detected by the central receiver control unit, the identification code is made available to the remainder of the system for further processing.
For Safety of Life applications, where an alarm condition may signify a health or security emergency, the RF wireless system must also comply with more stringent regulations, such as Underwriters Laboratories regulation UL864. This regulation requires that supervisory messages be transmitted at a reduced power level below that of the alarm message (i.e. normal, non-supervisory signals) by a minimum of 3 dB. Equivalent means may be utilized as long as the transmission of alarm messages comprises an effective power margin over that of the periodic transmission of supervisory messages sent by each transmitter in the alarm system.
Therefore, it would be advantageous if the alarm system would transmit supervisory messages at a power level below that of alarm messages, thereby ensuring that the transmission of alarm messages would have an effective margin over that of the periodic transmission of the supervisory message from each transmitter in the alarm system. Such a feature would be most beneficial if it could be applied without substantial modification to existing receivers already in commercial use.
Additional considerations may be found to hinder the implementation of such a concept such as overall cost, and various system parameters comprising message formats and protocols and repetition of messages. The ideal implementation would be transparent to such parameters that could very well differ from country to country or even among different manufacturers.
Therefore, it would be advantageous if the alarm system would provide a cost effective means for reducing the transmitted signal power of supervisory messages and also transmit alarm messages at full power, which is independent of such parameters as message format, repetition of messages, etc. Such a system would likely require differentiation between supervisory and alarm message in the transmitter prior to transmission.